Impregnation of a graphite sheet with a sealant

ABSTRACT

A method for sealing graphite plates formed from mechanically processed graphite sheets of exfoliated graphite particles is provided. The graphites sheets are infused with a sealant to fill about 90 volume percent of the pores contained in the sheet. Upon curing the sealant, a substantially gas impermeable graphite plate is provides. Such a plate is useful in fuel cell construction.

This application claims the benefit of No. 60/172,131, filed Dec. 17,1999.

FIELD OF THE INVENTION

The present invention relates to a flexible graphite sheet impregnatedwith a sealant. More particularly, the present invention relates to theinfusion of the sealant into the porosity of the graphite sheet prior tomechanical deformation of the sheet to form an article of manufacture.

BACKGROUND OF RELATED TECHNOLOGY

Natural graphites are made up of layered planes of hexagonal arrays ofnetworks of carbon atoms and typically exist in the shape of flakes innature. These layered planes of hexagonally arranged carbon atoms aresubstantially flat and are oriented so as to be substantially paralleland equidistant to one another. The substantially flat, parallelequidistant layers of carbon atoms are joined together by weak Van derWaals forces. These natural graphites are soft and brittle and aretypically difficult to form into a shape due to cracking along theselayered planes. Such characteristics of graphites are well known tothose skilled in the art, see, e.g., U.S. Pat. No. 5,149,518.

Natural graphites, however, may be formed into flexible sheets bycompressing exfoliated graphite particles. Exfoliated graphite particlesare formed by expanding the natural graphite flakes. In this expansionprocess, natural graphite flakes are intercalated by dispersing theflakes in a solution containing an oxidizing agent, for instance, amixture of nitric and sulfuric acid. After the flakes are intercalatedexcess solution is drained from the flakes and after washing with water,the intercalated graphite flakes are dried. Upon exposure to hightemperature, for instance 1,090-1,370° C. (2,000-2,500° F.), theparticles of intercalated graphite expand in dimension as much as 80 to1000 or more times their original volume in an accordion-like fashion inthe direction perpendicular to the layered planes of the hexagonallyarranged carbon atoms of the constituent graphite particles.

The exfoliated graphite particles are then compressed or compactedtogether, in the absence of any binder, so as to form a flexibleintegrated graphite sheet of desired thickness and density. Thecompression or compaction is carried out by passing a thick bed ofexpanded particles between pressure rolls or a system of multiplepressure rolls to compress the material in several stages into sheetmaterial of desired thickness.

The sheet material formed from the exfoliated graphite particles, unlikethe original graphite flakes, can be formed and cut into various shapes.The compression operation flattens the expanded graphite particlescausing them to somewhat engage and interlock. The compressionreorientates many of the carbon atoms from the perpendicular,accordion-like arrangement back into layered, parallel planes.Nevertheless, some carbon atoms remain in substantially nonparallelplanes. These carbon atoms in the nonparallel planes increase theporosity of the sheet as compared to natural graphite, having parallelplanes of carbon atoms, provide engagement among parallel planes ofcarbon atoms to provide flexibility to the sheet, and allow mechanicalshaping without substantial cracking. Furthermore, as the degree ofcompression increases, the degree of reorientation of carbon atoms fromnonparallel planes into layered, parallel planes also increases,especially near the exterior surfaces of the sheet.

The density of the compressed exfoliated product is typically in therange of about 0.08 to 1.4 g/cc (5 to 90 lbs/ft³) which is lower thanthe density of natural graphite (or fully compressed graphite) having abulk density of about 2.2 g/cc (140 lbs/ft³). As the density of agraphite product increases, the porosity of the graphite producttypically decreases. Porosity, P, is defined as the fraction of thetotal volume of a porous substance that is occupied by the pores of thesubstance, as shown below in Formula I.

P=V _(P) /V _(T),  (I)

where V_(P) is the pore volume and VT is the total volume.

The pore volume, V_(P), of a porous material is the total volume, V_(T),less the volume of 30 the fully compressed bulk material, V₀, or

V _(P) =V _(T) /V ₀.  (II)

The porosity of a porous substance may also be expressed in terms ofdensities, as shown below in Formula III.

P=1−D _(P) /D ₀,  (III)

where D_(P) is the density of the porous material and D₀ is the densityof the fully compressed material.

From Formula III, the porosity of the compressed product is about 0.96and to about 0.36 for products having a density of about 0.08 and 1.4g/cc, respectively. The porosity of a fully compressed material is zerobecause such a fully compressed material does not have pore volume. Thisabove-calculated porosity is often referred to as true porosity becausethe volume of both open and sealed pore spaces are included. Apparentporosity is a measurement of just the open-pore space which isaccessible to a fluid, such as nitrogen or mercury. The volume of suchopen pore space is then substituted for V_(P) in Formula I.

A higher density or a lower porosity product is typically too stiff foruse as flexible sheet graphite and is typically too mechanically weak tosurvive mechanical shaping processes. Some applications require higherimpermeability or greater mechanical strength than is typical forcompressed graphite sheets. For instance, anode and cathode fluid-flowplates used in a fuel cell should be substantially impermeable togaseous reactants and products, such as hydrogen and oxygen, to avoidundesired leakage of the reactants and products. For these applications,the graphite sheet is mechanically processed into a graphite plate, andthen a sealant is impregnated into the graphite plate sheet to seal theplate. Such a graphite plate without an impregnated-sealant is toomechanically weak and not sufficiently impermeable to gaseous reactantsand products for use as a fuel cell plate because of its internalporosity. The impregnation of the sealant into the graphite plates forthese applications, however, is often difficult to achieve because suchmechanical processing alters the graphite sheets by reorienting thecarbon atoms from nonparallel to layered, parallel planes therebyinhibiting sealant infusion.

These layered, parallel planes tend to block access of the internalporosity. Furthermore, as the density of the graphite sheet increases,e.g., greater than about 1.0 g/cc (62 lbs/ft³), impregnation of asealant into internal porosity becomes more difficult because the outersurface of the plate is characterized by a greater number of layered,parallel planes of carbon atoms. Impregnation of these denser sheets isoften quite time consuming, thus increasing the manufacturing costsassociated with such plates.

One technique for impregnating a dense graphite sheet is disclosed inU.S. Pat. No. 4,729,910. The disclosed technique is to de-aerate thesheet and to apply a sealant under multiple steps of reduced pressureand ambient pressure to facilitate the movement of the sealant into theporosity of the sheet. The technique suffers the disadvantage ofrequiring multiple, expensive pressure reducing steps and using asealant dissolved in organic solvents to reduce the viscosity of thesealant to permit entry of the less viscous sealant mixture into theporosity of the sheet. The organic solvent is removed under reducedpressure conditions before the sealant is heat cured. This technique notonly requires multiple pressure reducing steps to infuse the sealant,but also results in no more than about 20% by weight take-up of sealant,leaving many of the internal pores unfilled with the sealant.

U.S. Pat. Nos. 5,885,728 and 5,902,762 disclose a technique whereceramic fibers are incorporated into dense graphite sheets to facilitatesubsequent sealant infusion. This technique, however, suffers from thedisadvantage of introducing impurities, such as the ceramic fibersthemselves, which can be harmful to certain applications, such as fuelcells where the surfaces of the graphite sheets are coated or proximalto precious metals, such as platinum. Also, only up to about 20% byweight take-up of sealant is achieved, leaving many of the internalpores unfilled with the sealant.

Despite these efforts, a need exists for sealing dense graphite sheetswithout introducing impurities potentially harmful to the finalapplication and without requiring multiple, complex processing steps.Moreover, a need exists for infusing greater amounts of sealant intographite sheets to provide a graphite plate that is substantiallyimpermeable to light gaseous materials, for instance hydrogen gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a method of the present invention for infusingsealant into a flexible graphite sheet.

FIG. 2A is a perspective view of a low density, flexible graphite sheet.

FIG. 2B is a perspective view of the sheet of FIG. 2A that has beenmechanically deformed to form a graphite plate with a pattern therein.

FIG. 2C is a cross-sectional view of the sheet of FIG. 2A.

FIG. 2D is a cross-sectional view of the plate of FIG. 2B.

FIG. 3 is a schematic of a fuel cell containing graphite plates of thepresent invention.

FIG. 4 is a cross-sectional view with a partial cutaway of the fuel cellof FIG. 2 showing a continuous fluid channel for a fluid-flow fieldplate on a graphite plate.

FIG. 5 is a partial cross-sectional view of the continuous fluidchannels of the fuel cell of FIG. 2.

SUMMARY OF THE INVENTION

The present invention relates to a method of impregnating a graphitesheet with a sealant and mechanically deforming the sealant-impregnatedsheet to form a mechanically strong graphite plate. The method of thepresent invention is useful in filling up to about 95% by volume of theinternal pores of the sheet prior to the mechanical deformation. Thesealant within the pores of the mechanically deformed sheet is cured toprovide a substantially gas-impermeable plate that is mechanicallystrong.

A low density graphite sheet having a graphite density of about 0.08 toabout 0.8 g/cc is provided for sealant infusion. An anaerobic-curing ora heat-curing sealant that is capable of absorption into the internalpores of the sheet is selected. This sealant is contacted with the lowdensity sheet to obtain a 95% by volume void fill of the internal pores.The sealant-impregnated sheet is mechanically compressed to form agraphite plate which is substantially gas impermeable after the resin iscured by a free radical mechanism.

Useful sealant include phenolic resins, vinyl resins, silicone resins,acrylic resins, epoxy resins and the like. (Meth) acrylic resins areparticularly useful. Such sealants can be thermally or anaerobicallycured by free radical mechanisms.

In one aspect of the present invention, a substantially gas impermeablegraphite plate is provided. The graphite plate of the present inventionhas up to about 95% by volume of its pores filled with a curablesealant, which results in a substantially gas impermeable graphiteplate. Such plate is useful as a fuel plate in a fuel cell.

DETAILED DESCRIPTION OF THE INVENTION

Flexible graphite sheets that are formed from exfoliated graphiteparticles have greater internal porosity or a greater amount of poresthan natural graphites. Although the present invention is described as aflexible graphite sheet, other forms are suitable with the presentinvention. For example, exfoliated graphite particles may be suitableprocessed into other shapes, such as, but not limited to, a cone, acube, a cylinder, a disk, a prism, a parallelepiped, a sphere, and thelike. As the graphite sheet is formed into a denser product, theinternal pores are typically inaccessible because of increased surfacelayers and constituent layers of carbon atoms below the surface that aresubstantially parallel to that surface of the sheet. These layers blockthe flow of a sealant into the pores of the sheet. Sealant will,however, flow readily within the sheet in directions parallel to opposedparallel planar surfaces if the sealant can first penetrate past theparallel surface atoms.

As used herein, the phrase “flexible graphite sheets” and its variantsrefer to an article of manufacture formed from compression of exfoliatedgraphite particles without a resin.

The present invention provides a method of sealing a graphite sheet byproviding access for fluid communication of a sealant to these internalpores. The fluid communication permits the flow of a sealant into theporosity of the graphite sheet. Upon curing the sealant, a substantiallygas impermeable graphite sheet is obtained. Such a sealed sheet isuseful as an anode or cathode fuel plate in a fuel cell.

FIG. 1 is a schematic of the method of the present invention for sealinggraphite plates.

At step 10, a graphite form, such as a flexible graphite sheet, isselected. The flexible graphite sheet, which has an internal porousportion between opposed exterior, outer planar surfaces, may be formedby an exfoliation process and is also commercially available from theUCAR Carbon Technology Corporation, located in Danbury, Conn., under thetrade name Grafoil®. A flexible graphite sheet can be made or isavailable in a wide range of densities. For instance, exfoliatedgraphite particles can be formed into flexible graphite sheets havinginternal pores resulting in a graphite density of about 0.08 to 1.4 g/cc(5 to about 90 lbs/ft³). Higher density materials are more difficult toimpregnate with a sealant than lower density materials due to a greateralignment of the carbon atoms in the layered, parallel groups,especially at or near an outer surface of the sheet. Graphite sheets ofhigh porosity or low density with a graphite density of about 0.08 toabout 0.8 g/cc (5 to 50 lbs/ft³) are useful with the present invention.Graphite sheets having a graphite density of about 0.08 to about 0.5g/cc (5 to 31 lbs/ft³) are also useful with the present invention. Asused herein, the phrase “graphite density” and its variants refer to adensity of a flexible graphite sheet without having any sealant infusedinto the internal pores of the sheet.

At step 12, a curable sealant is selected. Useful sealants includeheat-curable and anaerobic-curing sealants, such as Resinol® 90C and RTCsealants, which are commercially available form the Loctite Corporation,Rocky Hill, Conn. These useful sealants are further described herein.

At step 14, the curable sealant of step 12 is contacted with at leastone of the outer planar surfaces of the flexible graphite sheet of step10. The sealant communicates from the outer surface into the internalpores of the sheet because the low density graphite sheet of step 10 hasaccessible porosity at its external surfaces to provide fluidcommunication for a sealant into its internal pores. The term “fluidcommunication” and its variants refer to, but are not limited to, fluidimpregnation, fluid transmission, fluid flow, fluid permeation, and thelike.

The sealant may be applied to the graphite sheet after the sheet isplaced under a vacuum to remove air from the porosity of the sheet. Theremoval of the air from the pores facilitates the subsequent infusion ofsealant. Such removal of air under vacuum conditions, however, is notrequired to practice the method of the present invention, but isdesirable for increasing infusion rates or reducing time requirementsfor infusion. Furthermore, the sealant may be applied under ambient orpositive pressure conditions. A positive pressure often helps to drivethe sealant into the pores.

At step 16, the sealant-impregnated sheet of step 14 is mechanicallyprocessed to form a graphite plate. Mechanical deformation, such ascompression, is a useful process for forming the plate. Furthermore, asdescribed further herein, such mechanical deformation can also includethe impressing of a pattern onto a surface of the plate.

The mechanical deformation typically results in a more denser sheet. Forexample, a graphite sheet with a graphite density of about 0.08 to about0.5 g/cc may be mechanically deformed resulting in a compressed sheet orplate with a graphite density greater than 1.0 g/cc. Furthermore,portions of the mechanically deformed sheet may be more resistant tosealant infusion than other portions. These aspects are illustrated inFIGS. 2A-2D.

FIG. 2A is a schematic of a flexible graphite sheet 100. Desirably, theflexible graphite sheet 100 has a graphite density of about 0.08 toabout 0.5 g/cc. Flexible graphite sheet 100 may be mechanically deformedto form graphite plate 120, which is schematically depicted in FIG. 2B.Graphite plate 120 may have a pattern, such as channel 124, withinplanar surface 122. Graphite plate 120, including channel 122, may beformed through use of a compression plate (not shown) bearing of reliefpattern of channel 122. Such a compression plate may be compressed ontoplanar surface 102 of flexible graphite sheet 100.

FIGS. 2C and 2D are cross-sectional views of flexible graphite sheet 100and graphite plate 120, respectively, taken along respective 2C—2C and2D—2D axes. Portions of the graphite plate 120 that form channel 122,such as channel portions 126 A, B and C, typically have a greatergraphite density than that of the flexible graphite sheet 100. Thehigher graphite density makes sealant infusion more difficult becauseof, in part, a greater alignment of carbon atoms in layered groupsparallel to channel portions 126A, B and C.

Furthermore, as depicted in FIGS. 2C and 2D, if the thickness offlexible graphite sheet 100, which is represented by vector D1, isreduced during mechanical compressions to yield a smaller thickness ofthe graphite plate 120, which is represented by vector D2, thensubstantial portions of planar surface 122 of graphite plate 120 mayalso be resistant to graphite infusion due to greater graphite densitiesand greater alignment of carbon atoms thereat.

Returning to FIG. 1, at step 18, the curable sealant of step 12 isdesirably cured through a free radical mechanism. Thermal and anaerobiccuring are useful with the present invention and are described below.Sealants may also be cured by ionic mechanisms.

Flexible graphite sheet, such as sheet 100, with a graphite density ofabout 0.22 g/cm³ was degassed under a vacuum of about 0.13 kilopascals(1 torr). The degassed sheet was contacted with a liquid porositysealant, such as Resinol® 90C. The sealant filled the internal pores toan extent of about 95% by volume. The 95% by volume void fillrepresented a weigh increase of 429% for the sealant-impregnated sheet.A denser sheet having a graphite density of about 1.14 g/cm³ wasimpregnated under similar conditions. The weight increase was only 22%,which represents a void fill of about 48% by volume.

It was expected that the extent of resin up-take in flexible graphitesheet would vary in inverse linear relationship to the graphite density.However, this is not the case. The graphite density of the first sheet(0.22 g/cm³) was 0.192 times (approximately ⅕) the graphite density ofthe second sheet (1.14 g/cm³), yet the lower density sheet absorbedapproximately 20 times the amount of sealant compared to the higherdensity sheet. This was 4 times greater than was expected from thedensity differences of the two materials.

The amount of sealant up-take can be controlled over a wide range byvarying the impregnation pressure, temperature, sealant viscosity,impregnation time of density of the sheet. It is generally desirable tohave a sealant up-take that fills as completely as possible the voidvolume of the sheet. The percent of voids filled can be estimated fromthe volumes and weights of the non-impregnated and thesealant-impregnated sheets, the density of the sealant (1.09 g/cm³) andthe density of bulk graphite (2.2 g/cm³). In general, it is exceedinglydifficult to fill more than about 70% by volume of the voids incompressed high-density grades of graphite sheets that are typicallyused for fuel cell construction. By impregnating the graphite sheetbefore it is compressed, almost quantitative filling of the voids hasbeen attained. In addition, the impregnation rates are significantlyenhanced by filling the voids before compressing the structure.

The impregnated low-density graphite sheet is soft and easily deformedwhile the sealant remains in its liquid form. On compression, excessliquid may, if necessary, be removed. The final amount of liquid sealantremaining in the sheet is determined, in part, by the load that isapplied during compression and by the time period during which that loadis maintained. Compression plates bearing a relief pattern of afluid-flow channels may be used to impress the required pattern into thesoft liquid-filled sheet prior to the curing of the resin.

The Resinol® 90C is a methacrylate-based thermosetting impregnationsealant that cures rapidly to give a hard impermeable matrix on heatingat temperatures of 90° C. or higher. The liquid sealant in the abovesheets is desirably cured by means of a hot press, which may be used tosimultaneously compress, imprint and cure the sealant composition.Curing times depend on the final plate thickness and the amount ofsealant up-take. Typically curing times range from a few minutes to 1hour over a temperature range of about 90 to about 180° C.Anaerobic-curing sealants, such as Resinol® RTC from the LoctiteCorporation, are also useful with the present invention. Such anaerobicsealants may be cured at ambient temperatures in the substantial absenceof oxygen.

In another aspect of the present invention, a flexible graphite plateinfused with a sealant for use as a fuel cell plate is provided. Thepresent invention, however, is not limited to fuel cell plates, but canalso be used in other applications where high strength graphite orhighly impermeable graphite is desirable. For example, a graphite gasketfor a high pressure application may require greater strength andimpermeability, as achieved by sealant impregnation, than a graphitegasket for a low pressure application.

The flexible graphite sheet 100 of the present invention is useful as afluid-flow field plate for use in a fuel cell. FIG. 3 shows,schematically, the basic elements of an electrochemical fuel cell, suchas fuel cell 200. Electrochemical fuel cells convert fuel and oxidant toelectricity and reaction product. The fuel cell 200 includes a membraneelectrode assembly (“MEA”) 210 consisting of a solid polymer electrolyteor ion exchange membrane 212 disposed between two electrodes 214A, 214Cformed of porous, electrically conductive sheet material, typicallycarbon fiber paper. The MEA 210 contains a layer of catalyst (notshown), typically in the form of finely comminuted platinum, at eachmembrane/electrode interface 216A, 216C to induce the desiredelectrochemical reaction. The electrodes 214A, 214C are electricallycoupled to provide a path for conducting electrons between theelectrodes to an external load (not shown).

At anode 218, the fuel permeates the porous electrode material ofelectrode 214A and reacts at the catalyst layer (not shown) atmembrane/electrode interface 216A to form cations, which migrate throughthe ion exchange membrane 212 to cathode 220. At the cathode 220,oxygen-containing gas reacts at the catalyst layer (not shown) atmembrane/electrode interface 216C to form anions. The anions formed atthe cathode 220 react with the cations to form a reaction product.

In electrochemical fuel cells employing hydrogen as the fuel andoxygen-containing air (or substantially pure oxygen) as the oxidant, thecatalyzed reaction at the anode 218 produces hydrogen cations (protons)from the fuel supply. The ion exchange membrane 212 facilitates themigration of hydrogen ions from the anode 218 to the cathode 220. Inaddition to conducting hydrogen ions, the ion exchange membrane 212isolates the hydrogen-containing fuel stream from the oxygen-containingoxidant stream. At the cathode 220, oxygen reacts at the catalyst layer(not shown) at membrane/electrode interface 216C to form anions. Theanions formed at the cathode 220 react with the hydrogen ions that havecrossed the ion exchange membrane 212 to form liquid water as thereaction product. The anode and cathode reactions in hydrogen/oxygenfuel cells are shown in the following equations:

Anode reaction: H₂→2H⁺+2e ⁻  (IV)

Cathode reaction: ½O₂→2H⁺+2 e ⁻→H₂O  (V)

The MEA 210 is disposed between two electrically conductive plates, suchas fluid-flow field plates 222A, 222C, each of which has at least oneflow passage 224A, 224C contained therein. These fluid-flow field plates222A, 222C are typically formed of compressed, exfoliated graphite. Theflow passages 224A, 224C direct the fuel and oxidant to the respectiveelectrodes, namely, the anode 218 on the fuel side and the cathode 220on the oxidant side. In a single cell arrangement, fluid-flow fieldplates are provided on each of the anode and cathode sides. The platesact as current collectors, provide support for the electrodes, provideaccess channels for the fuel and oxidant to the respective anode andcathode surfaces, and provide channels for the removal of water formedduring operation of the cell.

Two or more fuel cells 200 can be connected together, generally inseries but sometimes in parallel, to increase the overall power outputof the assembly. In series arrangements, one side of a given plateserves as an anode plate for one cell and the other side of the platecan serve as the cathode plate for the adjacent cell. Such a seriesconnected multiple fuel cell arrangement is referred to as a fuel cellstack (not shown), and is usually held together in its assembled stateby tie rods and end plates. The stack typically includes manifolds andinlet ports for directing the fuel and the oxidant to the anode andcathode flow field channels.

FIG. 4 is a cross-sectional view of a partial cutaway of the fuel cell200 taken along the 4—4 axis showing fluid-flow field plate 222C. Thefluid-flow field plate 222C includes a single continuous fluid-flowchannel 224C which has a fluid inlet 252C and a fluid outlet 250C. Fluidinlet 252C is connected to a source of oxidant (not shown). Continuousflow channel 224C traverses in a plurality of passes a major centralarea of fluid-flow field plate 222C, which corresponds to theelectrocatalytically active region of the cathode 220. Fluid field flowplate 222A has a similarly connected fluid-flow channel 224A, but itsfluid inlet is connected to a fuel source.

FIG. 5 is a partial cross-sectional view of the fuel cell 200, furtherdetailing the fluid-flow channels 224A, 224C. Fluid-flow channels 224A,224C are separated by walls 225A, 225C, respectively. The fluid-flowchannels 224A, 224C are typically 1.5 mm deep and 1-1.5 mm wide andextend to cover the electrode area of the fuel cell 200. The walls 225A,225C are typically 1-1.5 mm inch thick. The fluid-flow channels 224A,224C are formed by a mechanical deformation process, such as stamping,pressing, milling, molding and the like. A compression plate bearing arelief pattern of the fluid-flow channel is useful to impress thepattern in the flexible graphite sheet. The density of the fluid fieldplates 222A, 222C before mechanical deformation is approximately 0.1 to0.5 gm/cc (5 to 32 lbs/ft³) and after stamping the density typicallyexceeds 1.1 g/cc (69 lbs/ft³). Fluid field plates 222A, 222C are theflexible graphite sheet 100 of the present invention sealed with asealant, as described above.

Curable sealants useful with the present invention include any suitablesealant type such as phenolic resins, vinyl resins, silicone resins,acrylic resins, epoxy resins and the like. However, the presentinvention is particularly useful with (meth)acrylic resins.(Meth)acrylic resins are useful in porosity impregnation applicationsdue to their highly advantageous viscosity characteristics and rapidcurability in anaerobic cure and/or heat-curing formulations.Illustrative commercially available impregnation sealing compositionswhich may be utilized in the practice of the present invention includeResinol® 90C sealant (a registered trademark of Loctite Corporation,Rocky Hill, Conn.), a heat-curable (meth)acrylic resin; and Resinol® RTCsealant (a registered trademark of Loctite Corporation, Rocky Hill,Conn.), an anaerobic sealant composition curable at ambient temperaturesin the substantial absence of oxygen. These resins may be accompanied byother conventionally-used composition components, such as polymerizationinitiators, catalysts, plasticizers and the like.

Desirably, the sealant has a polymerizable component with a majority ofpolyfunctional (meth)acrylate esters (hereinafter, poly(meth)acrylateesters). These polyfunctional esters produce cross-linked polymers,which serve as effective and durable sealants, adhesives and coatings.While various (meth)acrylate esters may be used, desirablepoly(meth)acrylate esters include those with the following generalformula:

wherein R¹ represents a radical selected from the group consisting ofhydrogen, lower alkyl of from 1 to about 4 carbon atoms, hydroxyalkyl offrom 1 to about 4 carbon atoms and

R² is a radical selected from the group consisting of hydrogen, halogen,and lower alkyl of from 1 to about 4 carbon atoms; R³ is a radicalselected from the group consisting of hydrogen, hydroxyl, and

and m may be 0 to 12, and desirably from 0 to about 6; n is equal to atleast 1, e.g., 1 to about 20 or more, and desirably between about 2 toabout 6; k is 1 to about 4; and p is 0 or 1.

The polymerizable poly(meth)acrylate esters corresponding to the abovegeneral formula are exemplified by, but not restricted to, the followingmaterials: di-, tri- and tetraethyleneglycol dimethacrylate,dipropyleneglycol dimethacrylate; polyethyleneglycol dimethylacrylate(PEGMA); di(pentamethyleneglycol) dimethacrylate; tetraethyleneglycoldiacrylate; tetraethyleneglycol di(chloracrylate); diglyceroldiacrylate; diglycerol tetramethacrylate; tetramethylene dimethacrylate;ethylene dimethacrylate; and neopentylglycol diacrylate. Combinationsand derivatives of these polyfunctional materials are contemplated.

Monofunctional (meth)acrylate esters (esters containing one(meth)acrylate group) are also useful. The most common of thesemonofunctional esters include the alkyl esters such as laurylmethacrylate. Many of the lower molecular weight alkyl esters are quitevolatile, and frequently it is more desirable to use a higher molecularweight homolog, such as decyl methacrylate or dodecyl methacrylate, orany other fatty acid acrylate esters, in (meth)acrylate-basedimpregnating compositions.

When monofunctional (meth)acrylate esters are employed in the presentinvention, it is desirable to use an ester which has a relatively polaralcohol moiety. Such materials are less volatile than low molecularweight alkyl esters and, in addition, the polar group tends to provideintermolecular attraction in the cured polymer, thus producing a moredurable seal. Desirably the polar group is selected from the groupconsisting of labile hydrogen, heterocyclic ring, hydroxy, amino, cyano,and halogen polar groups. Typical examples of compounds within thiscategory are cyclohexylmethacrylate, tetrahydrofurfuryl methacrylate,hydroxyethyl acrylate (HEMA), hydroxypropyl methacrylate (HPMA),t-butylaminoethyl methacrylate, and chloroethylmethacrylate.Combinations of monofunctional (meth)acrylate are contemplated.

The sealants of the present invention may be anaerobically curablethrough a free-radical mechanism, with an initiator being presenttherein, or an initiator system comprising a redox polymerizationinitiator (i.e., an ingredient or a combination of ingredients whichproduce an oxidation-reduction reaction, resulting in the production offree radicals). Suitable initiators include peroxy materials, e.g.,peroxides, hydroperoxides, and peresters, which are capable of inducingpolymerization of the sealant compositions in the substantial absence ofoxygen, and yet not induce polymerization as long as oxygen is present.Organic hydroperoxides are the desirable peroxy materials with t-butylhydroperoxide and cumene hydroperoxide being particularly useful withthe anaerobic-curing compositions.

In addition to initiator components, sealants useful with the presentinvention may include various initiator accelerators, as for examplehydroperoxide decomposition accelerators, when hydroperoxides are usedas cure initiators in the sealant material. Typical examples ofpotentially suitable accelerators include: tertiary amines such astributyl amine; sulfimides such as benzoic sulfimide (or saccharin);formamide; and compounds containing transition metals, such as copperoctanoate.

The useful sealants may also be heat-curable compositions through afree-radical mechanism, with a heat-cure initiator being presenttherein, or an initiator system comprising a redox polymerizationinitiator (i.e., an ingredient or a combination of ingredients which atthe desired elevated temperature conditions, e.g., from about 90 toabout 150° C. (194 to 302° F.), produce an oxidation-reduction reaction,resulting in the production of free radicals). Suitable initiators mayinclude peroxy materials, e.g., peroxides, hydroperoxides, andperesters, which under appropriate elevated temperature conditionsdecompose to form peroxy free radicals which are initiatingly effectivefor the polymerization of the heat-curable compositions.

Another useful class of heat-curing initiators comprises azonitrilecompounds which yield free radicals when decomposed by heat. Heat isapplied to cure the composition, and the resulting free radicalsinitiate polymerization of the heat-curable composition.

For example, azonitrile may be a compound of the formula:

wherein R⁴ is a methyl, ethyl, n-propyl, iso-propyl, iso-butyl orn-pentyl radical, and R⁵ is a methyl, ethyl, n-propyl, iso-propyl,cyclopropyl, carboxy-n-propyl, iso-butyl, cyclobutyl, n-pentyl,neo-pentyl, cyclopentyl, cyclohexyl, phenyl, benzyl, p-chlorbenzyl, orp-nitrobenzyl radical or R⁴ and R⁵, taken together with the carbon atomto which they are attached, represent a radical of the formula

wherein m is an integer from 3 to 9, or the radical

Compounds of the above formula are more fully described in U.S. Pat. No.4,416,921, the disclosure of which hereby is incorporated herein byreference.

Azonitrile initiators of the above-described formula are readilycommercially available, e.g., the initiators which are commerciallyavailable under the trademark VAZO® from E. I. DuPont de Nemours andCompany, Inc. (Wilmington, Del.), including VAZO® 52 (R⁴=methyl,R⁵=isobutyl), VAZO® 64 (R⁴=methyl, R⁵=methyl), and VAZO® 67 (R⁴=methyl,R⁵=ethyl), all such R⁴ and R⁵ constituents being identified withreference to the above-described azonitrile general formula.

A desirable azonitrile initiator is 2,2′-azobis(iso-butyronitrile) orAZBN.

The azonitrile may be employed in the inventive heat-curablecompositions in concentrations on the order of about 500 to about 10,000parts per million (ppm) by weight, desirably about 1000 to about 5000ppm.

Other (meth)acrylic monomer-based impregnant compositions of aheat-curable character may be employed in the broad practice of thepresent invention, including those disclosed in UK Patent Specifications1,308,947 and 1,547,801. As described in these references, the monomericimpregnant composition may contain suitable inhibitors serving torestrict or preclude the occurrence of polymerization of the monomer, attemperatures below those desired or recommended for heat-curing of theimpregnant composition.

The invention may be further understood with reference to the followingnon-limiting examples. Percent weights are per the total compositionunless otherwise specified.

EXAMPLES Example 1

A flexible graphite panel having a density of about 0.22 g/cm³,dimensions of about 6.0 cm×6.0 cm×0.48 cm and a mass of about 3.771 gwas placed and secured in an empty 1 liter vacuum impregnation chamber.The chamber was fitted with a pressure-compensating reservoir containingliquid porosity sealant Resinol® 90C, supplied by the LoctiteCorporation. The chamber and reservoir were evacuated to a pressure ofabout 0.13 kilopascals (1 torr) at 23° C. (73° F.). After 15 minutes, asufficient volume (about 300 ml) of the liquid sealant was fed into thechamber to completely submerse the graphite panel. The pressure wasmaintained at about 0.13 kilopascals (1 torr) for an additional 15minutes after which the vacuum was released and the pressure restored toambient. The panel was held in the submersed state for an additional 1hour, then removed from the chamber and rinsed with water to remove alltraces of the sealant adhering to the external surface. The washed panelwas carefully dried with tissue paper and weighed. The final weight was19.965 g corresponding to a weight increase of 429% and a void fill of95% by volume.

Example 2

A flexible panel having a density of about 1.14 g/cm³, dimensions ofabout 6.0 cm×6.0 cm×0.076 cm and a mass of about 3.113 g was impregnatedunder the conditions described in Example 1. The final weight was 3.808g corresponding to a weight increase of 22% and a void fill of 48%. Thelower density panel of Example 1 exhibited substantially greater sealantinfusion as compared to the higher density panel of Example 2.

Example 3

The impregnated panel of Example 1 was placed between two glass panelsand gently compressed under hand pressure. During the compression, theimpregnated liquid was observed to ooze from the panel. The thickness ofthe panel decreased with increasing loss of sealant.

Example 4

The compressed, impregnated panel of Example 3 is rinsed with water toremove excess sealant from the panel's exterior surface. The panel iscured under elevated temperature conditions from about 90 to about 180°C. to provide a substantially gas impermeable graphite plate. Thesealant is cured within a few minutes or up to an hour. The curing timedepends upon the final plate thickness, the amount of sealant take-up,the curing temperature and the amount of heat-curing initiators andaccelerators.

Example 5

A flexible graphite panel, as described in Example 1, is impregnatedwith a liquid porosity sealant Resinol® RTC, supplied by the LoctiteCorporation, under conditions as described in Example 1. The sealup-take is an increase of 429% by weight, which represents a void fillof 95% by volume.

The impregnated panel is then compressed under conditions of Example 3.The compressed panel is rinsed to remove excess sealant. The rinsedpanel is held between two plates to exclude oxygen. Within about 20minutes the sealant is cured to provide a substantially gas impermeablegraphite plate. The curing time depends on the final plate thickness,the amount of sealant take-up, the degree of oxygen exclusion, and theamount of anaerobic-curing initiators and accelerators.

The invention being thus described, it will be clear to those persons ofskill in the art that many variations exist, which are not to beregarded as a departure from the spirit and scope of the invention. Allsuch variations are intended to be within the scope of the claims.

What is claimed is:
 1. A method for forming a graphite plate sealed witha sealant, said method comprising: a. providing a graphite sheet havingopposed planar surfaces defining an interior portion therebetween, saidsheet being formed from exfoliated graphite particles to provide saidinterior with pores and defining a graphite density of said sheet; b.providing a curable sealant; c. contacting said sealant with at leastone of said planar surfaces, allowing said sealant to communicatethrough said pores; d. compressing said sheet to form a graphite platehaving graphite density after compression, wherein said graphite densityis greater after compression than before compression; and e. curing saidsealant contained within said pores to seal said plate.
 2. The method ofclaim 1 wherein said graphite density before compression is from about0.08 to about 0.8 g/cc.
 3. The method of claim 1 wherein said graphitedensity before compression is from about 0.08 to 0.5 g/cc.
 4. The methodof claim 1 wherein said graphite density after compression is from about1.0 to 2.2 g/cc.
 5. The method of claim 1 wherein compressing furtherincludes forming a pattern in at least one of said planar surfaces. 6.The method of claim 5 wherein said plate is a fluid-flow plate of a fuelcell and further wherein said pattern is a continuous fluid-flowchannel.
 7. The method of claim 1 further comprising the step of fillingfrom about 70% to about 95% of said pores on a volume basis with saidsealant.
 8. The method of claim 1 wherein selecting said sealantincludes selecting a sealant from the group consisting of a phenolicresin, a vinyl resin, a silicone resin, an acrylic resin, an epoxyresin, and combinations thereof.
 9. The method of claim 1 furtherincluding the step of providing a free radical initiator in initiatecure of said sealant.
 10. The method of claim 9 wherein said freeradical initiator includes a heat-curing initiator to produce freeradicals by thermal decomposition to cure said sealant.
 11. The methodof claim 10 wherein said heat-curing initiator is selected from thegroup consisting of a peroxide, a hydroperoxide, a perester, anazonitrile and combinations thereof.
 12. The method of claim 9 whereinsaid free radical initiator includes an anaerobic-curing initiator toproduce free radicals upon the exclusion of oxygen to cure said sealant.13. The method of claim 12 wherein said anaerobic-curing initiator is aperoxy initiator selected from the group consisting of hydroperoxides,peroxides, peresters and combinations thereof.
 14. The method of claim12 wherein said anaerobic-curing initiator includes an anaerobicaccelerator selected from the group consisting of tributyl amine,benzoic sulfimide, formamide, copper octanoate and combinations thereof.15. The method of claim 1 wherein said selecting said sealant furthercomprises selecting a curable poly(meth)acrylate ester having theformula:

wherein R¹ represents a radical selected from the group consisting ofhydrogen, lower alkyl of from 1 to about 4 carbon atoms, hydroxyalkyl offrom 1 to about 4 carbon atoms and

R² is a radical selected from the group consisting of hydrogen, halogen,and lower alkyl of from 1 to about 4 carbon atoms; R³ is a radicalselected from the group consisting of hydrogen, hydroxyl and

m is 0 to about 12, n is equal to at least 1, k is 1 to about 4 and p is0 or
 1. 16. The method of claim 15 wherein said selecting said sealantfurther comprises selecting a monofunctional acrylate ester, saidmonofunctional acrylate ester being selected from the group consistingof lauryl methacrylate, cyclohexylmetharylate, tetrahydrofurfurylmethacrylate, hydroxyethyl acrylate, hydroxypropyl methacrylate,t-butylaminoethyl methacrylate, cyanoethylacrylate,chloroethylmethacrylate and combinations thereof.
 17. A graphite platecomprising: a. a graphite sheet formed from exfoliated graphiteparticles and having an internal, portion with pores between opposedplanar surfaces; and b. a curable sealant contained within greater than70% of said pores on a volume basis, wherein upon curing said sealantsaid graphite sheet is sealed with said sealant to form said plate. 18.The graphite plate of claim 17 wherein said sealant includes a sealantselected from the group consisting of a phenolic resin, a vinyl resin, asilicone resin, an acrylic resin, an epoxy resin, and combinationsthereof.
 19. The graphite plate of claim 17 further including a freeradical initiator to initiate cure of said sealant.
 20. The graphiteplate of claim 19 wherein said free radical initiator includes aheat-curing initiator to produce free radicals by thermal decompositionto cure said sealant.
 21. The graphite plate of claim 20 wherein saidheat-curing initiator is selected from the group consisting of aperoxide, a hydroperoxide, a perester, an azonitrile and combinationsthereof.
 22. The graphite plate of claim 19 wherein said free radicalinitiator includes an anaerobic-curing initiator to produce freeradicals upon the exclusion of oxygen to cure said sealant.
 23. Thegraphite plate of claim 22 wherein said anaerobic-curing initiator is aperoxy initiator selected from the group consisting of hydroperoxides,peroxides, peresters and combinations thereof.
 24. The graphite plate ofclaim 22 wherein said anaerobic-curing initiator includes an anaerobicaccelerator selected from the group consisting of tributyl amine,benzoic sulfimide, formamide, copper octanoate and combinations thereof.25. The graphite plate of claim 17 wherein said sealant furthercomprises a curable poly(meth)acrylate ester having the formula:

wherein R¹ represents a radical selected from the group consisting ofhydrogen, lower alkyl of from 1 to about 4 carbon atoms, hydroxyalkyl offrom 1 to about 4 carbon atoms and

R² is a radical selected from the group consisting of hydrogen, halogen,and lower alkyl of from 1 to about 4 carbon atoms; R³ is a radicalselected from the group consisting of hydrogen, hydroxyl and

m is 0 to about 12, n is equal to at least 1, k is 1 to about 4 and p is0 or
 1. 26. The graphite plate of claim 25 sealant further comprising amonofunctional acrylate ester, said monofunctional acrylate ester beingselected from the group consisting of lauryl methacrylate,cyclohexylmetharylate, tetrahydrofurfuryl methacrylate, hydroxyethylacrylate, hydroxypropyl methacrylate, t-butylaminoethyl methacrylate,cyanoethylacrylate, chloroethylmethacrylate and combinations thereof.27. The graphite plate of claim 14 wherein said graphite plate iscompressed to form a fluid-flow plate of a fuel cell.
 28. The graphiteplate of claim 24 wherein said fluid-flow plate includes a continuousfluid-flow channel extending through at least one of said planarsurfaces.